The present disclosure is related to the field of fetal monitoring. More specifically, the present disclosure is related to fetal heart rate monitoring with the detection of maternal artifacts.
Current methods to detect fetal heart rate (FHR) using Doppler ultrasound have difficulty when applied to obese patients as the ultrasound scanning depth of the solutions has a limited ultrasound penetration range that, when thickness of abdominal fat is taken into account, does not reach the heart of the fetus. Thus, the heart of a fetus with an obese mother is beyond the typical range of ultrasound sensitivity.
While ultrasound penetration depth can be increased either by increasing a duration of an ultrasound transmit pulse and a duration of a receive window or by increasing the duration of the receive window with a fixed transmit pulse, the increased penetration depth causes a decrease in the signal to noise ratio in the received reflected ultrasound signals. The ultrasound beam is produced in a cone shape and reflects across a wider area of physiological structures the deeper an ultrasound wave penetrates before it is reflected and returned. Therefore, ultrasound fetal heart rate monitors with extended penetration depth create a risk to pick up competing physiological signals of the mother, rather than the heart activity of the fetus. Abdominal ECG has been used to acquire fetal ECG signals in obese patients due to these challenges noted above for Doppler ultrasound FHR monitoring. However, the quality of the fetal heart rate determined using fetal ECG through abdominal ECG is typically lower than the quality of fetal heart rate detection experienced in the clinical setting for non-obese maternal patients using Doppler ultrasound. The abdominal ECG requires that a plurality of electrodes be applied to the patient, such electrodes are at risk of producing poor quality signals if careful skin preparation or maintenance of the electrodes is not considered.
An additional effect of this problem associated with increased Doppler ultrasound depth range is confusion of the maternal heart rate (MHR) for the fetal heart rate. Due to the increased risk of picking up the maternal pulse in the abdominal vessels, the MHR signal may dominate the returned ultrasound signals. This can occur in a case wherein the fetal heart signal is weak or lost due to worsening fetal health or due to movement of the fetal heart beyond the ultrasound beam as a result of maternal or fetal movements. In these instances, when operating at an increased ultrasound depth range, the monitor may mistakenly identify the MHR instead of the FHR. Also, if the MHR is elevated, then the monitoring device may lock on the MHR and erroneously produce a result indicating fetal well-being, when in fact the fetal health may be deteriorating.
One previously proposed solution has been to incorporate an additional independent transducer of MHR. Such independent transducer has exemplarily been a pulse oximetry (SpO2) device incorporated into a tocodynamometer. The addition of a still further technology and transducer adds complexity and cost to systems and such combinations present further challenges due to the operation of competing transducers in the same space. Specifically, SpO2 is known to be sensitive to motion artifact and the tocodynamometer membrane can be a source of such motion artifacts, limiting the quality of the MHR determined by the SpO2 transducer.